The submarine pens built in WWII were some of the biggest concrete construction works of the war, and some of toughest targets throughout all of Europe. The idea of protecting submarines from attack was first explored in WWI with wooden shelters, a time when men were dropping bombs weighing a fell kilograms from aircraft by hand.
By WWII, aircraft were now carrying bombs weighing in the tons, and as such the levels of protection for the U-boats needed to be increased dramatically. Construction of submarine bunkers in Hamburg and Heligoland island began, with many more on the way. The enormous scale of these bunkers made it clear from early on this was above the abilities of the Navy, so the civil and military engineering establishment Organisation Todt was brought in to take over the work.
In areas where the population was less resistant to their occupiers, much of the labour was performed by local workers. Essentially slaves, these workers were subject to extreme inhumane treatment, with little food or water and often worked until they died. Aside from their vast size, submarine bunkers were complex in their design. They had to provide office spaces, medical facilities, accommodation for personnel, storage for submarine supplies and spares, ammunition, anti-aircraft defences, fuel and power generators.
Allied air raids often harassed the worksites, damaging equipment and materials, disruption construction. Unfortunately, local civilian areas around the pens were also subject to bombs in collateral damage, and many of the slave labourers died during these attacks.
The bunkers were designed to be immune to all Allied bombs, with some having roofs up to 8 m thick 26 ft. These were enough to resist Allied bombs, accept two; the Tallboy and Grand Slam bombs. These British bombs weighed 12, lbs and 22, lbs respectively. They would reach supersonic speeds as they fell, burrowing into a target or the ground before exploding, with a thickened steel casing.
